Over the last years the demand for high value petroleum products such as gasoline, diesel and jet fuel have been increasing, while the supply of light crudes has been declining, which has been offset by the production of heavy crudes. Heavy crude oils and residua are characterized by exhibiting high content of contaminants such as sulfur, nitrogen, metals and asphaltenes, besides having low yield of distillable fractions. These trends along with the nature of heavy oils emphasize the importance of processes to convert such streams into lighter and cleaner ones.
Catalytic hydroprocessing is one of the most used approaches to upgrade heavy oils and residua due to its flexibility for handling different petroleum fractions. During hydroprocessing of heavy fractions various reactions occur, such as hydrodemetallization (HDM), hydrodesulfurization (HDS), hydrodeasphaltenization (HDAs), hydrodenitrogenation (HDN) and hydrocracking, depending on the reaction system, type of catalyst and severity of the reaction. Commonly, down-flow fixed-bed reactors are employed for hydroprocessing. The reactor(s) are sequentially loaded with a series of catalysts (CoMo/NiMo) with different functionalities. The catalyst system in fixed-bed heavy oil hydroprocessing is generally composed of different types of catalysts designed for specific objectives such as HDS, HDM, HDN, mild hydrocracking (MHC) and Conradson Carbon (CCR) removal.
The main problem with fixed-bed heavy hydrocarbon hydroprocessing is the poisoning and fouling of the catalyst bed over time. To achieve economically acceptable catalyst cycle lives, commonly HDM guard reactors have been used to protect down stream catalysts; another option is to tailor the operating conditions in order to keep at low levels the catalyst deactivating reactions such as HDM and coking, such an operation implies keeping a balanced reactor pressure and temperature, the latter being the most essential variable for achieving an acceptable catalyst cycle length and the required product quality.
Hydroprocessing reactions are exothermic in nature, reason why an appropriate temperature control system is required. Commonly, the required reactor temperature profile is adjusted by dividing the total catalyst volume into several beds separated by inter-bed zones that allow introducing and mixing quench streams. Traditionally, hydrogen has been the quench fluid of choice in most of these processes; however, liquid quenching is also practiced.
Hydrogen quenching has the advantage of replenishing hydrogen consumption of the preceding bed and diluting the concentration of hydrogen sulfide and ammonia; its availability depends on the hydrogen-to-oil (H2/oil) ratio along the reactors, which depends primarily on the compressor capacity within the plant. High H2/oil ratios increase quench availability, however, also increase compressor requirements, besides increasing superficial gas velocity across the reactor, which can produce excessive pressure drops.
When hydrogen quenching is limited, liquid quenching, particularly with cold hydrocarbon streams, can be advantageous due to their higher heat capacity and lower compression cost. Various liquid quench based process schemes have been proposed, such schemes can be classified into split- or multiple-feed processes and product recycle processes.
The split-feed processes are characterized by previously fractionating the feed and introducing each fraction into the reactor in descendent order of boiling ranges, from the top of the reactor to the last bed at the bottom, in such a way that the light fractions act as quench streams and also are processed in the following catalytic bed.
In the case of product recycle processes, a previously cooled portion of the reactor effluent is recycled in order to be used as quench stream and to provide a second pass opportunity to unreacted species.
The following patent documents describe split-feed processes:                U.S. Pat. No. 3,728,249 (Antezana et al.) proposes a method for introducing middle distillates with different compositions into a multi-bed hydrotreating reactor;        U.S. Pat. No. 5,290,427 (Fletcher et al.) provides a method for upgrading FCC gasoline, which involves separating the feed into a light olefin rich fraction and a heavy sulfur rich fraction, in order to introduce the heavy fraction at the top of the reactor for an extended contact time, whereas the light fraction is introduced towards the end of the reactor for reducing olefin saturation;        U.S. Pat. No. 5,603,824 (Kyan et al.) reveals a process for upgrading waxy hydrocarbon streams in a multi-bed reactor by subjecting the heavy fraction of the feed to hydrocracking and dewaxing in the top bed of the reactor, and the light fraction, in admixture with the bed effluent from the top bed, to hydrodesulfurization in the subsequent beds;        U.S. Pat. No. 6,299,759 (Bradway et al.) discloses an alternative method for quenching hydrotreating reactors by splitting the liquid feed into several streams of the same composition in order to be injected at several lengths of the reactor;        U.S. Pat. No. 6,583,186 (Moore), U.S. Pat. No. 6,589,415 (Smith et al.), and U.S. Pat. No. 6,656,342 (Smith et al.) provide process schemes for hydroprocessing Fischer-Tropsch products comprising liquid quenching; in this case, the heavy fraction is subjected to hydrocracking conditions at the top beds of the reactor, whereas the rest of the fractions are subjected to hydrodesulfurization in the following beds; and        U.S. Patent Publication No. 2003/0010678 (Kalnes) proposes a method for vacuum gasoils, comprising a first HDS/HDN stage, in which lighter hydrocarbons are used for quenching, followed by a HCR stage.        
As for the product recycling processes, the following patent documents are available:                U.S. Pat. No. 3,425,810 (Scott) proposes several schemes for introducing and withdrawing liquid streams between catalyst beds in order to provide stripping to these streams and/or to reintroduce them, after being previously cooled, as quench;        U.S. Pat. No. 3,489,674 (Borst) reveals a method for hydroprocessing hydrocarbons characterized by recycling part of the liquid product stream as quench;        U.S. Pat. No. 5,492,617 (Trimble et al.) illustrates a quenching method for an upflow reactor for the hydroprocessing of heavy hydrocarbons; and        U.S. Pat. No. 7,014,750 (Boger et al.) provides a method to hydrogenate pyrolysis gasoline, in which a recycle stream of hydrogenated gasoline is employed as quench.        
The processes based on liquid quenching do not disclose nor suggest their application to the hydroprocessing of heavy hydrocarbons, such as heavy and extra heavy crude oils, residues, and their blends with light crude oils, because the are difficult to handle with.